This invention relates to sense circuits and in particular to sense circuits capable of producing unambiguous outputs when comparing signals of approximately equal value.
In some applications, the state or condition of an element or device can be best determined only by comparing it with a like element or device operated as a reference. For example, a problem exists in sensing the state of the metal-nitride-oxide-semiconductor (MNOS) transistors of a memory array because their threshold voltages (V.sub.T) shift as a function of the number of times and the sequence in which the MNOS transistors are set to a high (V.sub.TH) or a low (V.sub.TL) threshold voltage condition.
Since the V.sub.T 's may shift above or below their original setting, comparison with a fixed reference is not satisfactory. A solution to this problem resides in designating an MNOS transistor as a reference and subjecting it to similar stresses to which the other MNOS transistors are subjected. This ensures that the characteristics of the reference transistor will track those of the other MNOS transistors and solves the problem of having a suitable reference. However, a problem still exists in that, the element being sensed and the reference element, due to their similar characteristics, produce like signals when, for example, they are set to the same V.sub.T condition.
Distinguishing between two like signals is difficult. A balanced sense circuit looking at two virtually identical input signals would produce an indeterminate output signal or, if it included regenerative feedback, would flip to one of its stable states in a relatively random manner producing an erroneous or ambiguous indication of the condition of the element being sensed. For the example of the MNOS transistors, the transistor being sensed and the reference transistor could both be at the same V.sub.T condition. However, small differences in the characteristics of the transistors and/or small amounts of noise in the system might cause the signal produced by the transistor being sensed to be greater (or less) than the signals produced by the reference transistor. A balanced sense circuit comparing the two signals would then produce an erroneous indication of the true state of the transistor being sensed.